A polymer electrolyte fuel cell (PEFC) has a stacked structure constituted by a plurality of single cells that have a power generation function. Each of single cells has a membrane electrode assembly (MEA) including (a) a polymer electrolyte membrane, (b) a pair of catalyst layers to interpose the polymer electrolyte membrane therebetween, and (c) a pair of gas diffusion layers (GDL) to disperse supply gas. The MEA in one single cell is electrically connected to another MEA in an adjacent single cell via a separator. Then, a fuel cell stack is constituted by a plurality of the single cells that are stacked on top of each other. The fuel cell stack thus obtained functions as a power generation means available for various purposes. In the fuel cell stack, a separator functions to electrically connect the adjacent single cells to each other, as described above. In addition, the surface of the separator facing the MEA is generally provided with a gas flow path. Such a gas flow path functions as a gas supply means to supply fuel gas and oxidant gas to an anode and a cathode, respectively.
The following is a simple explanation of a power generation mechanism of the PEFC. At the time of the operation of the PEFC, fuel gas (such as hydrogen gas) is supplied to an anode side, and oxidant gas (such as air and oxygen) is supplied to a cathode side. As a result, electrochemical reactions represented by the following reaction formulae proceed at the anode and cathode sides, respectively, so as to generate electricity.[Math 1]Anode reaction: H2→2H++2e−  (1)Cathode reaction: 2H++2e−+(½)O2→H2O  (2)
As a material constituting a separator for a fuel cell required to have electrical conductivity, metal, carbon and electrical conductive resin are conventionally used. Among those materials, a separator constituted by carbon or electrical conductive resin is required to be relatively thick in order to maintain a certain level of intensity of the separator after the formation of a gas flow path formed thereon. Accordingly, the total thickness of a fuel cell stack using such a separator becomes thicker. The increase in thickness of the stack is not preferable since a compactification of the PEFC for a vehicle is required.
On the other hand, a separator constituted by metal has relatively high intensity. Therefore, a thickness of the metal separator can be reduced to some extent. In addition, due to excellent electrical conductivity, there is an advantage of decreasing a contact resistance to the MEA by using the metal separator. However, such a metal separator has possibilities of a decrease in electrical conductivity caused by corrosion, and also a power reduction derived from the decrease in electrical conductivity. Consequently, the metal separator is required to have improved resistance to corrosion while excellent electrical conductivity is ensured.
Patent Literature 1 discloses a technique to form a metal layer such as Ti and a carbide layer thereof on one surface of a metal substrate of a metal separator, followed by forming a carbon layer constituted by graphitized carbon on the metal layer and the carbide layer.
Patent Literature 2 discloses a technique to form an oxide film of a substrate of a metal separator between the substrate and an electrical conductive thin film so as to form a middle layer constituted by metal elements or metalloid elements.
Patent Literature 3 discloses a separator in which a carbon-based film containing a composite metal oxide is formed on a substrate.